I should say at this point, if you have the heat set gun,
of course you should use it. If you've never tried it out before
this tutorial, where you can find out how to
embellish a scarf with them. But, if you'd like to know
the quick and dirty way of doing it, try this trick:
Cut a little square of fabric as a pressing cloth.
Turn your iron onto high (usually the cotton setting)
with no steam. Carefully place your crystal on your
mask, then lay the pressing cloth on top, and finally
the tip of the hot iron. Take care not to shift your
crystal when you do this. You'll want to hold your iron
on there for about 30 seconds. When you remove it, wait
at least 10 seconds to touch your crystal, as the glue
may still be hot and not set. Once you've let it cool,
test to see if the glue took hold.